Anapsida turtles and their extinct kin

The earliest reptiles, including Hylonomus and Paleothyris, were small-sized insectivores that lived among the vegetative debris and tree stumps of the Middle Carboniferous forest. They are numbered among the first anapsids, the most ancient lineage of reptiles. Hylonomus and Paleothyris had a superficial resemblance to today's lizards, but they were not directly related to them. Hylonomus and Paleothyris were probably quick on their feet and well adapted for life on land.

Between the Late Carboniferous and the Early Permian, the anap-sid reptiles diverged in several directions. One surprising development was a return to the water of a group of Southern Hemisphere anapsids known as mesosaurs. Measuring about 3.5 feet (1 m) long, the mesosaur body was long and reminiscent of that of a crocodile. The limbs of the mesosaurs retained the basic design of land-based tetrapods, but their digits were webbed to aid in swimming. The mesosaur tail was also modified for swimming: It had tall, flat sides down its length. Most curious was the long and narrow mesosaur jaw. The upper and lower jaws were lined with a mesh of needlelike teeth that the animal used to trap and filter small arthropods, fishes, and other swimming creatures out of the water.

In contrast to the small, lizardlike anapsids were the hulking pareiasaurs. Found only in rocks dating from the Late Permian, these animals were large, heavy creatures comparable in size to the hippopotamus. Measuring between 6 and 10 feet (1 and 3 m) long, pareiasaurs had powerful, stumplike limbs and a sturdy, humped back protected by armor studs. The skull was short and blunt, with an array of bony frills jutting out from the cheeks. This extraordinary armor plating suggests that pareiasaurs needed protection from some of the equally large predatory synapsid reptiles that shared their habitat.

Pareiasaurs had perfected the eating of plants through several adaptations to their skull and jaws. The pareiasaur skeleton shows evidence of a large hump over the neck, an indication of strong neck and jaw muscles. The blunt, serrated, leaf-shaped teeth of these animals clearly indicate that they fed on soft vegetation. The best-known pareiasaur is Scutosaurus—the "shield lizard"—from Late Permian deposits in western Russia. Scutosaurus measured about eight feet (2.4 m) long and had a height at the peak of the back of about six feet (1.8 m).

As successful as the anapsids were during the latter part of the Paleozoic Era, their longevity eventually would be whittled down to the survival of a scant few members. These survivors included the armor-plated turtles and tortoises, the earliest forms of which arose during the Late Triassic Period. A contributing factor in the demise of many anapsids probably was the rise of the synapsids, once known as mammal-like reptiles. The synapsids included among their members the largest predators of their time.